Usually hackers exploit a certain condition of the system, it can be as simple as using an addition operation (simple plus) or as complex as writing a piece of code to hack a system.
recently have started bug bounty program in which they will pay you for identifying the security loopholes, in short you can earn too, a lot. Most of giant companies like Google, Yahoo, Facebook etc. A lot of computer/cellphone users want to hack cause its fun and somehow you get fame through it.
If I say that everyone wants to hack, I won’t be lying.